The ornamental design for a spa, as shown and described.
FIG. 1 is a perspective view of a spa of my new design;
FIG. 2 is a front elevation view thereof, with the rear, left side and right side elevation views being identical;
FIG. 3 is a top plan view thereof; and,
FIG. 4 is a bottom plan view thereof.
Portions of the spa, such as the interior of the spa and the supports for the spa, shown in broken lines in the figures are for illustrative purposes only and form no part of the claimed design.